mingw: fix isatty() after dup2()

We newly handle isatty() by special-casing the stdin/stdout/stderr file
descriptors, caching the return value. However, we missed the case where
dup2() overrides the respective file descriptor.

That poses a problem e.g. where the `show` builtin asks for a pager very
early, the `setup_pager()` function sets the pager depending on the
return value of `isatty()` and then redirects stdout. Subsequently,
`cmd_log_init_finish()` calls `setup_pager()` *again*. What should
happen now is that `isatty()` reports that stdout is *not* a TTY and
consequently stdout should be left alone.

Let's override dup2() to handle this appropriately.

This fixes https://github.com/git-for-windows/git/issues/1077

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.h

@@ -475,6 +475,9 @@ int mingw_raise(int sig);
 int winansi_isatty(int fd);
 #define isatty winansi_isatty
 
+int winansi_dup2(int oldfd, int newfd);
+#define dup2 winansi_dup2
+
 void winansi_init(void);
 HANDLE winansi_get_osfhandle(int fd);
 

compat/winansi.c

@@ -470,6 +470,18 @@ static void die_lasterr(const char *fmt, ...)
 	va_end(params);
 }
 
+#undef dup2
+int winansi_dup2(int oldfd, int newfd)
+{
+	int ret = dup2(oldfd, newfd);
+
+	if (!ret && newfd >= 0 && newfd <= 2)
+		fd_is_interactive[newfd] = oldfd < 0 || oldfd > 2 ?
+			0 : fd_is_interactive[oldfd];
+
+	return ret;
+}
+
 static HANDLE duplicate_handle(HANDLE hnd)
 {
 	HANDLE hresult, hproc = GetCurrentProcess();